1. Field of Invention
This invention relates to the design and construction of structures, specifically to structures with prefabricated deck units.
2. Prior Art
Full-depth precast concrete deck has gained popularity as an accelerated construction method. Use of full-depth precast concrete deck allows for the deck concrete and reinforcement to be placed in a controlled environment, improving the quality of the deck. Since the units are prefabricated, they can be delivered to a site and erected quickly.
Structures using full-depth precast concrete deck typically consist of a plurality of longitudinally spaced concrete units supported by longitudinal load-carrying members. These members are usually a single girder or multiple girders or beams. This member or members can be comprised of various materials including steel, concrete or fiber-reinforced plastic.
When no longitudinal post-tensioning is used in conjunction with a precast concrete slab deck, the use of cast-in-place joints between precast deck units is required. The cast-in-place joint requires extensive fieldwork and the uncompressed joint typically exhibits long-term maintenance and durability problems.
An improvement that has been made to precast concrete decks is to introduce longitudinal post-tensioning. The post-tensioning can provide a compression force across the deck joints, whereby improving the durability of cast-in-place joints. With the exception of the technology proposed in U.S. Pat. No. 7,475,446 B1, all current precast deck construction employs internal post-tensioning, wherein post-tensioning ducts or sheaths are embedded inside the concrete deck. The current practice of using internal post-tensioning has several disadvantages, including:                a. The extensive ductwork in the precast concrete deck units requires the ducts to be placed very accurately so that they will align with the ducts in the adjacent unit.        b. Duct coupling is required at the joints between the precast concrete deck units, which is time consuming and a labor intensive process. If a duct is not coupled properly, jointing materials can leak into the duct and cause duct blockage. This can result in significant construction delays and construction quality problems.        c. The internal post-tensioning is vulnerable to corrosion, particularly in climates where deicing chemicals are used. These chemicals can penetrate through the concrete and corrode the post-tensioning steel, especially at locations where the post-tensioning ducts are coupled.        
U.S. Pat. No. 7,475,446 B1 provides a solution to introduce post-tensioning external to the deck, using a method to transfer longitudinal compression to the deck units when all deck units are non-composite with the longitudinal load-carrying members and with longitudinal tensioning elements anchored at one more specially designed deck end units. The proposed method discussed herein also provides a solution to introduce post-tensioning external to the deck, but utilizes composite deck connection units in the transfer of longitudinal compression to the deck units and does not necessarily require anchorage of the tensioning elements into the deck units, as the tensioning elements can also be anchored in the longitudinal load-carrying members themselves or other locations.